Characterization of polyurethane and a silk fibroin-polyurethane composite fiber studied with NMR spectroscopies

Recently, Bombyx mori silk fibroin (SF) has played a crucial role in biomedical applications. However, due to the highly ordered β-sheet structure in SF, the flexibility of SF fiber is relatively poor, which limits its application in biomaterials. In this paper, a SF-polyurethane (PU) composite fiber was prepared to increase the flexibility of SF fiber. PU prepared by a reaction of mainly a copolymer of polyhexamethylene carbonate diol and polycaprolactone diol (PHC/PCL-diol) with 4,4’-diphenylmethane diisocyanate (MDI) was used. First, the 1 H and 13 C solution NMR peaks of PU were assigned to groups in detail. In addition, the 13 C solid-state NMR spectrum of PU was analyzed. Then, regenerated SF-PU composite fiber was prepared by the wet spinning method. The elongation at break of the fiber increased by 1.3-1.8 times compared with that of regenerated SF fiber, although there was no significant increase in tensile strength. The main reason for the increased flexibility of the SF-PU composite fiber is the increase in the fraction of random coils in SF, which was clearly observed by 13 C solid-state NMR. A newly synthesized polyurethane (PU) was used to increase the flexibility of Bombyx mori silk fibroin (SF) fiber by making a composite of SF and PU, which is expected to increase applications of SF fiber in biomaterials. Namely, the elongation at break of the fiber increased by 1.3–1.8 times compared with that of regenerated SF fiber. The main reason for the increased flexibility of the SF-PU composite fiber is the increase in the fraction of random coils in SF, which was clearly observed by 13 C solid-state NMR.